The bright red color of menstrual blood may be caused by physiological factors such as larger menstrual flow or organic lesions of the uterus, such as uterine fibroids, endometrial polyps and so on. 1. Larger menstrual flow: If a woman has a larger menstrual flow, her menstrual flow will appear bright red, which is a normal physiological phenomenon. 2. Organic uterine lesions: uterine fibroids are a common phenomenon, especially submucosal fibroids, which usually cause menstrual blood to be bright red, because submucosal fibroids grow into the uterine cavity, occupying a larger area than the uterine lining, and therefore cause an increase in menstrual flow. 3. Endometrial polyps: Endometrial polyps can also lead to increased menstrual flow, causing menstrual periods to be bright red. If the menstrual period is abnormal, then it is recommended to seek medical attention to avoid aggravation of the situation.